The present invention relates to a semiconductor laser control method and a semiconductor laser control apparatus to stabilize the output of outgoing radiation power of an apparatus that records or reproduces information on an optical information recording medium using a semiconductor laser as a light source.
For an apparatus that reproduces or records information on an optical information recording medium using a laser light source, a semiconductor laser is used as the light source for the purposes of reducing the size, reducing power consumption and improving mass-productivity, etc.
FIG. 9 shows a current-optical output characteristic of a semiconductor laser. A current with which a semiconductor laser starts oscillation as a laser and starts to output power is called a xe2x80x9cthreshold current Ithxe2x80x9d and the efficiency of optical output with respect to a drive current equal to or greater than the threshold current Ith is called xe2x80x9cdifferential quantum efficiency xcex7. Since this characteristic is extremely unstable, control is normally exercised to stabilize output power by detecting outgoing radiation power of a semiconductor laser and providing feedback for a semiconductor laser drive circuit.
FIG. 8 is a block diagram showing an outlined configuration of a conventional semiconductor laser control apparatus. In FIG. 8, reference numeral 31 denotes a semiconductor laser; 32, a power detection means for detecting power output from the semiconductor laser 31; 33, an outgoing radiation power target value; 34, a power control means for controlling outgoing radiation power of the semiconductor laser 31 to a desired power level by comparing the output value of the power detection means 32 and target value 33; 35, a CPU; and 36, a control signal.
The principle of operation of the semiconductor laser control apparatus with such a configuration is explained.
Here, suppose the outgoing radiation power of the semiconductor laser 31 is Pout, sensitivity detected by the power detection means 32 is K, the output signal of the power detection means 32 is X, the target value 33 is REF, an error signal obtained by subtracting the output signal X of the power detection means 32 from the target value 33 in the power control means 34 is Y, an amplification factor of the power control means 34 is G, and an output current of the power control means 34 is I. Then, if a feedback loop is configured only focused on a DC current, the following are obtained:
Y=REFxe2x88x92Xxe2x80x83xe2x80x83Equation 1
I=Gxc2x7Yxe2x80x83xe2x80x83Equation 2
xe2x80x83Pout=xcex7xc2x7(Ixe2x88x92Ith)xe2x80x83xe2x80x83Equation 3
X=Kxc2x7Poutxe2x80x83xe2x80x83Equation 4
If X, Y and I are erased from the Equations 1, 2, 3 and 4, Pout is obtained as follows:
Equation 5       P    out    =            REF      K        -                  1                  1          +                      G            ·            η            ·            K                              ·              REF        K              -                  1                  1          +                      G            ·            η            ·            K                              ⁢              I        th            
Generally, in a linear feedback control system, only the second term of the right side of the Equation 5 exists as a steady-state deviation of the control system. However, the semiconductor laser 31 has a current-optical output characteristic as shown in FIG. 9 and the threshold current Ith brings about the third term of the right side of the Equation 5. If the Equation 5 is solved with respect to the target value REF, REF is given in the following Equation 6:
REF=Kxc2x7Pout+Hxe2x80x83xe2x80x83Equation 6
where H is a correction term and is given:
H=Pout/Gxc2x7xcex7+Ith/Gxe2x80x83xe2x80x83Equation 7
Therefore, to obtain the desired power, it is only necessary to set to the target value (REF) determined by the Equations 6 and 7. Instead of obtaining the target value by the Equations 6 and 7, the conventional method used to adjust the target value so that the semiconductor laser outgoing radiation power Pout would consequently reach the desired power through power adjustment carried out in the manufacturing process.
However, the conventional control method had a basic problem that variations of Ith or xcex7 due to variations in the operating temperature or deterioration of the semiconductor laser would produce an error in output power.
When the operating temperature of the semiconductor laser increases from a temperature T1 to T2, the threshold current Ith increases and the differential quantum efficiency xcex7 decreases as shown in the current-optical output characteristic in FIG. 9. Since the correction term H of the Equation 7 includes the threshold current Ith and the differential quantum efficiency xcex7, the value of the correction term H will also change. Therefore, the Equation 6 also changes, with the result that the outgoing radiation power Pout of the semiconductor laser 31 changes.
Since the adjustment of the target value in the Equation 6 takes place in the manufacturing process, the correction term H is a value at the time of adjustment. Suppose the threshold current of the semiconductor laser during the process adjustment is Ith1; differential quantum efficiency is xcex71; outgoing radiation power of the semiconductor laser during the adjustment is Pout1; and the correction term is H1, then, the predetermined value (REF1) is given from the Equations 6 and 7:
REF1=Kxc2x7Pout1+H1xe2x80x83xe2x80x83Equation 8
and
xe2x80x83H1=Pout1/Gxc2x7xcex71+Ith1/Gxe2x80x83xe2x80x83Equation 9
If Equations 8 and 9 are solved with respect to Pout1 the following expression is obtained:
Equation 10       P    out1    =                              G          ·                      η            1                    ·          K                          1          +                      G            ·                          η              1                        ·            K                              ·                        REF          1                K              -                            η          1                          1          +                      G            ·                          η              1                        ·            K                              ⁢              I        th1            
On the other hand, the characteristic of the semiconductor laser changes under temperature conditions different from those at the time of process adjustment or deterioration of life due to operation for an extended period of time. Suppose the threshold current is Ith2 and differential quantum efficiency is xcex72 at this time. In this case, the outgoing radiation power Pout2 of the semiconductor laser is obtained by replacing Ith1, and xcex71 in the Equation 10 by ith2 and xcex72 respectively.
Equation 11       P    out2    =                              G          ·                      η            2                    ·          K                          1          +                      G            ·                          η              2                        ·            K                              ·                        REF          1                K              -                            η          2                          1          +                      G            ·                          η              2                        ·            K                              ⁢              I        th2            
Here, suppose the error in the outgoing radiation power of Pout2 corresponding to Pout1 is xcex94P=(Pout2xe2x88x92Pout1),
Equation 12                               Δ          ⁢                      xe2x80x83                    ⁢          P                =                  xe2x80x83                ⁢                                            (                                                                    G                    ·                                          η                      2                                        ·                    K                                                        1                    +                                          G                      ·                                              η                        2                                            ·                      K                                                                      -                                                      G                    ·                                          η                      1                                        ·                    K                                                        1                    +                                          G                      ·                                              η                        1                                            ·                      K                                                                                  )                        ·                                          REF                1                            K                                -                                                  xe2x80x83                ⁢                  (                                                                      η                  2                                                  1                  +                                      G                    ·                                          η                      2                                        ·                    K                                                              ⁢                              I                th2                                      -                                                            η                  1                                                  1                  +                                      G                    ·                                          η                      1                                        ·                    K                                                              ⁢                              I                th1                                              )                    
Since the total loop gain Gxc2x7xcex71xc2x7K and Gxc2x7xcex72xc2x7K are set to values sufficiently greater than 1, the following approximations can be used:
1+Gxc2x7xcex71xc2x7K≈Gxc2x7xcex71xc2x7Kxe2x80x83xe2x80x83Equation 13
1+Gxc2x7xcex72xc2x7K≈Gxc2x7xcex72xc2x7Kxe2x80x83xe2x80x83Equation 14
If this is applied to the Equation 12, xcex94P is obtained as follows:
xcex94P=xe2x88x92(Ith2xe2x88x92Ith1)/Gxc2x7Kxe2x80x83xe2x80x83Equation 15
As an example, the outgoing radiation power error AP is obtained for a semiconductor laser with a wavelength of 650 nm. Here, suppose a process adjustment is carried out at 25xc2x0 C. and the threshold current is Ith1, and differential quantum efficiency is xcex71 at that time. On the other hand, suppose the actual operation is performed at 60xc2x0 C. and the threshold current is Ith2 and differential quantum efficiency is xcex72 at that time. Typical temperature variations are numerically expressed as follows:
Ith1=50 (mA)xe2x80x83xe2x80x83Equation 16
xcex71=0.7 (W/A)xe2x80x83xe2x80x83Equation 17
Ith2=100 (MA)xe2x80x83xe2x80x83Equation 18
xcex72=0.6 (W/A)xe2x80x83xe2x80x83Equation 19
Suppose a total loop gain Gxc2x7xcex71xc2x7K is:
xe2x80x83Gxc2x7xcex71xc2x7K=100xe2x80x83xe2x80x83Equation 20
Then, Gxc2x7K is obtained from the Equations 17 and 20:
Gxc2x7K=100/0.7 =142.9 (A/W)xe2x80x83xe2x80x83Equation 21
xcex94P at this time is calculated as:
xcex94P=xe2x88x92(100xe2x88x9250)/142.9=xe2x88x920.35 (mW)xe2x80x83xe2x80x83Equation 22
From this, it is known that an error as great as xe2x88x920.35 mW is produced in the outgoing radiation power of the semiconductor laser.
By the way, it is assumed that the amplification factor G of the power control means 34 is constant, but there is also another control method of changing G according to a value xcex7 so that the total loop gain Gxc2x7xcex7xc2x7K is constant. In this case, suppose the amplification factor of the power control means 34 during a process adjustment is G1, the amplification factor of the power control means 34 when the semiconductor laser characteristic changes due to deterioration of life caused by temperature conditions different from those during the process adjustment and operation for an extended period of time is G2, and the total loop gain is a constant value A.
G1xc2x7xcex71xc2x7K=Axe2x80x83xe2x80x83Equation 23
G2xc2x7xcex72xc2x7K=Axe2x80x83xe2x80x83Equation 24
Since the detection sensitivity K of the power detection means 32 is a fixed value, G1 and G2 are changed according to the differential quantum efficiencies xcex71 and xcex72 as a consequence. In this condition, the outgoing radiation power error xcex94P is calculated as follows:
xcex94P=xe2x88x92(xcex72xc2x7Ith2xe2x88x92xcex71xc2x7Ith1)/(1+A)xe2x80x83xe2x80x83Equation 25
Substitution of the above described Equation 16 of the semiconductor laser of a wavelength 650 nm, Equations 17, 18, 19 and a value of the total loop gain A,
A=100xe2x80x83xe2x80x83Equation 26
into this Equation 25 yields:
Equation 27                               Δ          ⁢                      xe2x80x83                    ⁢          P                =                              -                          (                                                0.6                  xc3x97                  100                                -                                  0.7                  xc3x97                  50                                            )                                /                      (                          1              +              100                        )                                                  =                              -            0.25                    ⁢                      xe2x80x83                    ⁢                      (            mW            )                              
which is an improvement on the Equation 22, but there still is an error as great as xe2x88x920.25 mW.
Furthermore, the present example describes a characteristic change of the semiconductor laser caused by temperature variations, but similar changes in the threshold current Ith and differential quantum efficiency xcex7 also take place due to deterioration of life with time, thus producing an error of outgoing radiation power for the same reason.
Thus, the conventional control method poses a problem of inability to provide stable power control against temperature variations or deterioration of life.
The present invention has been implemented taking into account the above described problems of the conventional art and it is an objective of the present invention to provide a method and apparatus for semiconductor laser control capable of accurately stabilizing outgoing radiation power of a semiconductor laser to a desired value even when the characteristic of the semiconductor laser changes due to temperature variations or deterioration of life.
In order to attain the above objective, the present invention includes a power detection means for detecting outgoing radiation power of a semiconductor laser, power control means for controlling the outgoing radiation power of the semiconductor laser to a desired power level by comparing an output value of this detection means and a target value, target value correction means for correcting the above described target value, and can obtain the desired outgoing radiation power of the semiconductor laser with high accuracy even if the threshold current and differential quantum efficiency of the semiconductor laser change after adjustment of the outgoing radiation power of the semiconductor laser by correcting the target value based on a threshold current and differential quantum efficiency of the semiconductor laser through this target value correction means.
The present invention further includes a temperature detection means and can obtain the desired outgoing radiation power of the semiconductor laser with high accuracy also in case of temperature variations by correcting the target value by detecting a threshold current and differential quantum efficiency of the semiconductor laser according to temperature variations of the semiconductor laser.
The present invention further includes a semiconductor laser operation time measurement means and can obtain the desired outgoing radiation power of the semiconductor laser with high accuracy also in the case of deterioration of life by correcting the target value by detecting a threshold current and differential quantum efficiency of the semiconductor laser according to operation over time of the semiconductor laser.